The Katarchean

Abstract

The oldest (older than 3500 m.y.) supracrustals and plutonic rocks building up the basement of all of the remaining geologic formations belong to the Katarchean or Aldanian. This erathem is characterized by: (a) high-grade metamorphism of granulite and amphibolite facies (in most cases the rocks are the result of polymetamorphism); (b) ubiquitous, though irregular granitization and migmatization; (c) the supracrustals are principally represented by metavolcanics and metasedimentary rocks of less abundance, psephites are rare or absent; (d) absence of determinable organic remains; (e) absence of structural-facial and metamorphic zonation (in place to place it is faintly expressed); (f) grouping of linear and dome-shaped folds into large isometric or irregularly shaped systems; (g) dominance of plastic deformation. The above mentioned features of the erathem and some more will be examined below. The basement of the erathem is unknown. In many regions of the world the supracrustals and plutonic rocks are unconformably overlain by the Paleoprotozoic (Archeoprotozoic) greenstone strata, but their original relations are commonly masked by later thermal-tectonic processes.